The microtome with the designation Leica Ultracut UCT, on which the Leica FCS cooling chamber is mounted, has two illumination systems as standard equipment. Fluorescent lamps are mounted outside the chamber next to the objective of the stereomicroscope, and serve to illuminate the cut sections. The second illumination system is positioned, by means of light guides, directly beneath the knife. The bright gap produced by this illumination is helpful as the knife and specimen are brought close to one another.
The model MTX and MTXL microtome of the RMC company has an illumination system that is equipped similarly to the illumination system of the Leica company's microtome. Since the illumination of the fluorescent lamps is not sufficient, a fiber optic illumination system is offered as an accessory. Two goosenecks placed to the left and right of the knife are usual.
As already mentioned, illumination by means of fluorescent lamps has too little intensity to produce good visibility of the cut sections in the stereomicroscope. The illumination using a fiber optic system, on the other hand, can be brought very close to the knife and has sufficient intensity, but on the one hand it interferes with manipulation when the sections are removed from the knife, and on the other hand illumination with the fiber optic system causes ice crystals.
These ice crystals are produced because cooling chambers for ultramicrotomes are operated with liquid nitrogen. The liquid nitrogen produces nitrogen gas, which fills the chamber. Continuous formation of this gas causes it to flow steadily out of the cooling chamber. This continuous flushing with dry gas prevents the entry of moist air and thus the formation of ice deposits, with no need for a cover on the chamber (which is open at the top).
If, however, objects such as the fiber optic system are inserted from outside into the chamber, they penetrate through the boundary layer between the cold nitrogen gas and the air at room temperature. These objects introduce heat into the cold region and cause turbulence, and the cold gas no longer flows out steadily. Turbulence at the boundary layer with the moist air results in the precipitation of small ice crystals, which then also settle on the knife and reduce the service life of a knife between cleanings.